If neither the accelerator pedal nor the brake pedal is being operated by the driver of the motor vehicle, when the engine is running and a gear engaged—this gear must be appropriate for starting—, then in a large number of motor vehicles, through appropriate control of the drive train, a function is implemented which causes the motor vehicle to move slowly. In technical jargon, this function is also known by the name of “creeping”. This type of creeping function—hereinafter also referred to as creep mode—makes it easier to shunt or start on an incline.
Automatic transmissions generally have a hydrodynamic converter which, when the accelerator and brake pedals are not being operated, causes the motor vehicle to start rolling and to move at a moderate speed. In this case, the engine torque is increased by the converter when the engine is idling, which also produces increased tractive force on starting. Thus, from the vehicle viewpoint, when neither the accelerator pedal nor the brake pedal is being operated, a predetermined load-dependent wheel torque is made available in correlation with adjustment of the internal combustion engine's idling speed.
Hydrodynamic converters are, however, dissipative, and indeed increasingly so the greater the conversion. However, since operating in conversion mode results in higher fuel consumption, it is in many systems severely restricted with a view to optimizing efficiency—usually by using a bridging clutch—or completely different solutions are resorted to for the creeping or starting function.
From the published document EP 1 078 805 A1 a drive train controller is known which has a detection circuit, by means of which detection circuit the respective driving situations of the motor vehicle and the driver's characteristics are determined. A control unit for the clutch is adaptively adjusted when the motor vehicle is started to the driving situation and/or driver's characteristics determined. When the motor vehicle is started, signals with which the engine speed is controlled according to stored characteristics are transmitted by the control unit to the engine controller. A separate characteristic curve is also provided here for a creep mode.
Known drive train controllers for automating a creeping function are based upon selectively increasing the clutch torque when a starting gear is engaged in the transmission. Such creeping functions are referred to below as clutch-torque-based creeping functions. They relate to vehicles with an automated shift transmission and automated clutch. Both dry-running and wet-running variants are suitable for use as a clutch here. A disadvantage in such an implementation of the creeping function, however, is that the clutch torque represents the driver's wishes only indirectly. Since the complete drive train is located between the clutch system and the wheel drive of a motor vehicle, a change in the clutch torque can, depending on the operating conditions of the individual components of the drive train, bring about different changes in the wheel torque. This is, for example, the case if different gears are engaged in the transmission. When creeping, if the starting gear with the highest ratio (first gear) is selected, for example, then the resulting wheel torque is, given the same clutch torque, greater than in the gear with the second-highest ratio (second gear).
Where a creeping function is based on a clutch torque, consideration must therefore be given to the entire drive train when adjusting the clutch torque. Consequently, as the complexity of the drive train increases, the complexity of determining the clutch torque to be set also increases. However, a trend toward more and more complex drive systems can already be seen, e.g. through use of an additional drive unit in the form of a crankshaft starter generator alongside the conventional internal combustion engine.